Monitor on headphones while playback runs through the laptop speaker on a Mac

The direct answer: to hear the same audio on your headphones and your Mac's built-in speaker at the same time, build a Multi-Output Device in Audio MIDI Setup with both devices ticked, then set it as your system output. It's free and built in, and it mirrors every sound to both. The trade-offs are real — the master volume keys stop working and you balance each device by hand. If what you actually want is to monitor one app privately on headphones while everything else keeps playing out loud, that's per-app routing, and it needs a small third-party tool.

"Monitor on headphones while playback runs through the laptop speaker" hides two different requests, and picking the wrong one costs you an afternoon. One is mirroring: the exact same stream in your ears and out the speaker at once — a private check while the room still hears it, or listening along without cutting the speaker. The other is a split: the thing you're monitoring goes to the headphones, and unrelated audio stays on the speaker. macOS handles the first natively and ignores the second entirely. Sort out which you're after before you touch a single setting.

The native way: a Multi-Output Device (same sound, both devices)

macOS drives one output device at a time — the Sound settings pane and the volume keys move that single device, and everything the Mac plays goes there together. To make two devices sound at once you leave the plain Sound menu and build a combined device. Open Audio MIDI Setup (in Applications → Utilities), click the + in the bottom-left corner, and choose Create Multi-Output Device. Tick your Built-in Output (the laptop speaker) and your headphones, then set that new device as your system output from the menu bar or Sound settings.

That's it — playback now lands in your headphones and comes out of the laptop speaker at the same moment, identically. For "let me monitor this while the desk still hears it," it's exactly right, and it costs nothing. But there are two honest catches worth knowing before you rely on it.

The two catches nobody mentions until you hit them

The volume keys stop working. The moment a Multi-Output Device is your output, the master volume keys go greyed out. That's expected behavior, not a bug — macOS doesn't know which of the combined devices you mean to adjust, so it disables the single master control. To change levels you open Audio MIDI Setup and set each device's balance individually. Riding your headphone level up while the speaker stays put is no longer a keystroke; it's a trip into a utility window.

The two devices can drift out of sync. Inside a combined device, one output is the clock source and the others run at slightly different sample rates. Left alone, they slowly slide apart until you hear an echo between your ears and the speaker. The fix is built in: in Audio MIDI Setup, enable Drift Correction on the non-clock device (keep your built-in speaker as the primary/clock device at the top of the list). It's a documented, well-known step — you just have to know to flip it, or the echo creeps in.

Where a Multi-Output Device is the wrong tool

A Multi-Output Device mirrors everything. Every app on your Mac plays through both the headphones and the speaker, at the same relative level, with no idea which app is which. That's fine when you genuinely want the whole output duplicated. It falls apart the moment your real goal is narrower:

"I want to monitor this recording app on my headphones, but keep my music on the laptop speaker." "Send the video I'm reviewing to my ears, leave Slack and alerts on the speaker." "Let me hear the call privately while the speaker keeps playing background music for the room." None of those are mirroring — they put different audio on the two devices. A Multi-Output Device can't do that, because it has no concept of per-app output. That job is per-app routing, and it lives in third-party territory. For the general version — different apps on two outputs at once — the companion guide on using two audio outputs at the same time on a Mac, per app walks the whole thing.

The per-app version: monitor one app on headphones, keep the rest on the speaker

Per-app routing does what macOS won't: it assigns an output device to a single app, independent of the system default. You leave the default output on your built-in speaker, add one rule sending the app you're monitoring to your headphones, and both play at once — the monitored app in your ears, everything else out loud. No mirroring, no greyed-out volume, no drift correction to babysit, because you're not combining devices at all.

Two apps own this space, and they're honestly different. SoundSource from Rogue Amoeba is the deep option: per-app output routing plus per-app EQ and effects, a one-time $49. Fader does the routing in a clean list — set a device per app and it sticks across restarts — for a one-time $19.99, and it also gives every running app a volume fader you drive straight from the keyboard. That last part matters for monitoring: you can nudge the headphone app's level against the speaker mix with letter keys instead of digging through a utility. Here's how the approaches line up.

One honest note in Fader's own column: for pure mirroring — the identical stream to headphones and speaker with nothing else going on — you don't need Fader at all. The free Multi-Output Device is the right, simplest tool, and it would be silly to pretend otherwise. Fader earns its place when you want the split: one app monitored on headphones, the rest on the speaker, each level under your fingers. Same for Background Music, the free open-source mixer — it's great at per-app volume, but as of 2026 it keeps every app on the same output device, so it can't do the routing this split needs.

The exact setup for the split, step by step

With a per-app routing app the recipe is three steps and holds until you change it:

1. Leave your default system output on the built-in speaker — that's where music, Slack, alerts, and everything unrouted keeps playing. 2. Add one app route: pick the app you want to monitor (the recorder, the video, the call) and choose your headphones as its output. 3. Put the headphones on. The monitored app is now private in your ears; the rest of the Mac stays out loud on the speaker. Because nothing is combined into a Multi-Output Device, your volume keys keep working normally on the speaker output.

In Fader that's the Routing tab: the default output sits at the top, and each app route is a row — App → Device — from a dropdown. And since the same app hands every running app a keyboard fader, you can duck the speaker music a touch while you concentrate on the headphone monitor, all without leaving what you're doing. If your need is really the plain "one app in the headphones, everything else on the speakers" split, the focused walkthrough on sending one app to headphones while the rest stays on speakers covers exactly that.

Bottom line

If you want the same audio in your headphones and out the laptop speaker at once, a free Multi-Output Device in Audio MIDI Setup is the correct tool — just expect greyed-out volume keys and a quick Drift Correction toggle to stop the echo. If you want to monitor one app on headphones while the rest of your Mac keeps playing on the speaker, that's per-app routing, which macOS doesn't do on its own. SoundSource is the deep $49 option; Fader does the per-app routing plus keyboard-fast volume for $19.99 one-time, so you can monitor privately and still ride every level without opening a utility.